The present invention relates to novel binding peptides and, more particularly, to small peptides that bind to the serpin-enzyme complex or SEC receptor.
.alpha..sub.1 -antitrypsin (.alpha..sub.1 AT) is a single-chain, 55 kDa serum glycoprotein having 394 amino acids with no disulfide bridges and containing 3 oligosaccharide side chains. It is one of the most well characterized members of a family of serum proteins referred to as serpins. See, e.g., Huber and Carrell, Biochemistry 28, 8951-8971 (1990); Perlmutter and Pierce Amer. J. Physiol. 257, L147-L162 (1989). Members of this family include inhibitors of coagulation (ATIII, heparin cofactor II, protein C inhibitor), fibrinolysis (.alpha..sub.2 antiplasmin, plasminogen activator inhibitors I and II) and complement (Cl inhibitor). Several serpins such as .alpha..sub.1 antichymotrypsin and protease nexin I play a role in connective tissue turnover Non-inhibitory homologues such as corticosteroid-binding globulin and thyroid hormone-binding globulin bind their hormone ligands by a serpin-like mechanism [Pemberton et al., Nature 336, 257-258 (1988)] while ovalbumin and angiotensinogen may serve as substrates for their cognate enzymes [Wright, J. Biol. Chem. 259, 14335-14337 (1984)] . Although these proteins bear only 25-30% homology in overall primary structure there are much higher degrees of homology in specific regions. Each serpin which has been examined has a reactive center within an exposed loop acting as a pseudosubstrate for its cognate enzyme. There is a tight complex formed by serpin and enzyme, structural rearrangement of the serpin and cleavage at the reactive site peptide bond which may be variable in extent. See, e.g., Travis and Salvesen, Ann. Rev. Biochem. 52, 655-709 (1983).
Since the major physiologic target of .alpha..sub.1 AT is neutrophil elastase an enzyme capable of degrading many connective tissue matrix constituents, the serpin is thought to function as an inhibitor of connective tissue turnover This is supported by the fact that .alpha..sub.1 AT deficiency is associated with premature development of pulmonary emphysema. See, e.g., Crystal, J. Clin. Invest. 95, 1343-1352 (1990). Elastase also degrades many serum proteins and, thus, perturbations in the elastase-.alpha..sub.1 AT balance are believed to contribute to the defects in coagulation, fibrinolysis and complement activation which accompany systemic inflammatory states such as sepsis or adult respiratory distress syndrome See, e.g., Carrell, J. Clin. Invest. 77, 1427-1431 (1986). It has only recently been recognized that .alpha..sub.1 AT has other potential functions. When in complex with neutrophil elastase, .alpha..sub.1 AT possesses neutrophil chemoattractant properties [Banda et al., J. Biol. Chem. 263, 4481-14484 (1988); J. Exp. Med. 167, 1608-1615 (1988)] and mediates an increase in de novo biosynthesis of .alpha..sub.1 AT itself [Perlmutter et al., J. Clin. Invest. 81, 1774-1780 (1988); Perlmutter and Punsal, J. Biol. Chem. 263, 16499-16503 (1988)]. These observations have suggested that structural rearrangement of the .alpha..sub.1 AT molecule, during formation of a complex with elastase, exposes a domain that is recognized by a specific cell surface receptor, or receptors. In a recent report by Perlmutter et al., Proc. Natl. Acad. Science 87, 3753-3757 (1990), synthetic peptides of at least 16 amino acid residues and based on the sequence of a potentially exteriorly exposed and highly conserved region of the complex form of .alpha..sub.1 AT were tested as candidate ligands (carboxy-terminal fragment, amino acids 359-374). .sup.125 I Peptide 105Y (SIPPEVKFNKPFVYLI) bound specifically and saturably to a single class of receptors on HepG2 cells and human blood monocytes (K.sub.2 =4.0.times.10.sup.-8 ; 4.5.times.10.sup.5 plasma membrane receptors per cells) and mediated an increase in synthesis of .alpha..sub.1 AT. Binding of .sup.125 I peptide 105Y was blocked by .alpha..sub.1 AT-elastase complexes but not by the corresponding native proteins Furthermore, unlabelled peptide 105Y blocked binding of .sup.125 I.alpha..sub.1 AT-elastase complexes. These data indicated that at least part of the domain of .alpha..sub.1 AT corresponding to peptide 105Y was available for receptor binding and that structural rearrangement of .alpha..sub.1 AT during complex formation was a prerequisite for recognition by receptor. Binding of .sup.125 I peptide 105Y was also blocked by AT III-thrombin complexes, .alpha..sub.1 ACT-cathepsin G complexes and, to a lesser extent C1 inhibitor-Cls complexes but not by the corresponding native proteins. Thus, the responsible cell surface receptor was referred to as the serpin-enzyme complex or SEC receptor. Subsequent studies have shown that the SEC receptor mediates endocytosis and lysosomal degradation of .alpha..sub.1 AT-protease complexes [Perlmutter et al., J. Biol. Chem. 265, 16713-16716 (1990)].
The sequence of peptide 105Y which consists of the carboxy-terminal fragment, amino acids 359-374, of .alpha..sub.1 AT, converted to the three-letter abbreviations, is designated herein and in the accompanying Diskette as SEQ ID NO:1, and numbered from 1 to 16 in accordance with 37 CFR 1.821-825.